In the fields of video monitoring, video analysis, and so on, there is a case of, in accordance with a request from a request source, extracting a still image from a moving image and providing the request source with the extracted still image. In this case, a response time gets long in a case where a process of extracting a still image from a moving image is started at the time of request.
Then, as a related art relating to the present invention, it is proposed to extract still images from a moving picture and store the still images into a cache in advance and, when there is a request actually, retrieve a still image from the cache and provide a request source with the still image, as is disclosed in Patent Document 1, for example.    Patent Document 1: Japanese Unexamined Patent Application Publication No. JP-A 2013-090194
Because of the limit of cache capacity, it is difficult to store all still images extracted from a moving image into a cache. Therefore, there is a need to select still images to be stored into a cache from among a plurality of still images extracted from a moving image.
Regarding such selection, the abovementioned related art relating to the present invention employs a method as described below.
A still image extracted from a moving image by using the latest version of image processing program is stored into a cache. In a case where the still image extracted by using the latest version of image processing program is requested by a request source, the still image is retrieved from the cache and transmitted to the request source. In a case where a still image extracted by using a different version from the latest version of image processing program is requested by a request source, the still image is extracted from a moving image by using that version of image processing program and transmitted to the request source. In this case, the still image transmitted to the request source is stored into the cache. When the amount of data in the cache exceeds a given amount determined in advance, a still image with the earliest time of storage into the cache is erased from the cache.
However, according to the related art relating to the present invention, still images to be stored are determined without consideration for the cost of extraction of still images from a moving image, and therefore, a case will arise where still images with higher extraction cost among a plurality of still images extracted from a moving image are not cached. In other words, a case will arise where the cost of extraction of still images other than the stored still images is high.
Consequently, when there is a request for the still images other than the stored still images, it is difficult to extract the still images from the moving image at high speeds and at low load, and a response time is degraded.